1. Field of the Invention
The present invention relates to a fixing apparatus having a roller and a heat source, wherein a print medium to which an image has been transferred is held and conveyed by the roller to thereby fix the transferred image to the print medium with pressure and heat.
2. Description of the Related Art
In a conventional fixing apparatus having a roller with a heat source disposed therein, the heat source is positioned at the center of the roller, and heat radiated from the heat source is conducted from the inside of the roller toward the peripheral surface thereof. Accordingly, due to the conducted heat and a holding pressure generated between the roller and another roller associated therewith, the transferred image is fixed to the print medium.
This type of fixing apparatus has a defect in that, when the heat source positioned at the center of the roller starts to radiate heat, the temperature of the roller at the peripheral surface thereof cannot immediately rise to the temperature required for the fixing operation, and accordingly, a certain time must elapse before the fixing operation can be performed. Further, the heat radiated from the center of the roller heats the entire peripheral surface thereof to a constant temperature, regardless of the part of the surface first engaged in the fixing operation. Namely, the heat radiated from the heat source is dispersed throughout the roller, and thus it is not possible to start the fixing operation until a certain time has lapsed, which is uneconomical from the standpoint of time and energy consumption.
Furthermore, when the heat source is switched on, the temperature at a heating unit used for the fixing operation does not immediately rise to a temperature required for the fixing operation. Accordingly, a certain lapse of time must occur before the temperature required for the fixing operation after the heat source is switched on. Consequently, a warning lamp or the like must be lit to inform an operator that the temperature at the heating unit has risen to the temperature required for the fixing operation.
The heating unit temperature required to carry out the fixing operation without malfunctions is usually set within a certain range. Namely, overheating of the heating unit must be prevented, and therefore, the heat source is alternately switched on and off to maintain the temperature of the heating unit within a range required for the fixing operation.
In this case, however, if a reference temperature at which the warning lamp notifying the operator of the temperature of the heating unit is switched on or off is set at the same temperature as that at which the heat source is switched on or off, this will cause problems in that the lamp is repeatedly switched on and off regardless of whether or not the temperature at the heating unit is within the range required for the fixing operation. Therefore, a temperature completely different from the reference temperature at which the heat source is switched on or off must be set as the reference temperature at which the warning lamp is switched on and off.
In the conventional fixing apparatus, since a plurality of temperature reference values are set for the heating unit, the relationship between the magnitude of the respective values is determined for controlling the switching on or off of the heat source and the warning lamp. Therefore, the controlling of the temperature detecting circuit, the heat source, and the warning lamp becomes complicated and troublesome.